Osomatsu-san: Baby Oso?
by Misaki StupidSaru Yata
Summary: Choromatsu has finally had it with Osomatsu's irresponsible self and finally he tells the eldest Matsuno sextuplet his thoughts on it all. But, what happens when an unexpected accident happens and leaves him and Karamatsu as the eldest brothers for awhile? Warnings: Panic Attacks, Slight Violence, Amnesia. Set after episode 24 and 25 and Karamatsu Time.
1. Chapter 1

Chapter 1: The Argument:

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 _Author's Note: Hiii everyone! I know I haven't been updating like I usually do but, that is for the simple fact that I'm in Virginia on a one week vacation so I haven't picked up my laptop like I've wanted to._

 _On another note. Me and cuddlySoldier and another author have been working on a great pieces for Osomatsu-san while I'm still waiting for more ideas and summaries for fanfics that I will update and finish._

 **Story Summary: Choromatsu has finally had it with Osomatsu's irresponsible self and finally he tells the eldest Matsuno sextuplet his thoughts on it all. But, what happens when an unexpected accident happens and leaves him and Karamatsu as the eldest brothers for awhile?**

 **Disclaimer: I don't own anything. All credit goes to the original owners.**

 **ENJOY THE STORY!**

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Todomatsu walked into his house after a long day at work, and expected to see his shitty no good brothers just lazing on the floor the whole time. But, what he saw instead was a scene he never wanted to see again in his life.

Choromatsu and Osomatsu were glaring at each other as Karamatsu stood in the middle attempting to pacify whatever situation was going on between the third and first sons. Ichimatsu was attempting to keep Jyushimatsu calm, for it seemed the fifth son was close to breaking down the longer the fight dragged on.

"You shitty eldest brother!" Choromatsu yelled as he pushed against Karamatsu's firm hand that pushed against his chest, each time he tried to get closer to the other who seemed just as determined to get to the other the moment he could. "Why can't you just grow the fuck up?!"

"You shitty younger brother!" Osomatsu yelled back, hands curling into fists. "You think you're any damn better? All you fucking do is say you're looking for a job but, behind the job circular, you're goddamn fapping off to some idol shit! You hypocritical, tight-assed, virgin NEET!"

The room fell silent, save for Osomatsu harsh pants as he tried to regain his lost breath from screaming.

"..." Choromatsu stared at the older in shock. "...I hate you… In truth I think we all hate you!"

Todomatsu felt his stomach fall into his feet keeping him firmly grounded to the floor. His throat was suddenly dry as he watched his two older brothers fight, he's seen them fight with each other before, Hell, he was in most of those fights. But, they never escalated this badly.

In fact it seemed that those three words, were the words for Osomatsu to finally snap. His eyes narrowed in apparent anger, he stomped out of the house, pushing past Todomatsu like he wasn't even there.

The minute the door closed, Todomatsu fell to his knees, and a loud piercing wail hit the quiet air as the tension eased and Choromatsu slumped to the floor seemingly exhausted.

"You shouldn't have said that…." Todomatsu murmured dazedly as he looked at his older brother eyes confused. "Why would you say that?"

"What possessed you to say that Choromatsu-niisan?" Jyushimatsu wailed as he cried into Ichimatsu's shoulder, while Karamatsu took the other side to comfort the distraught sextuplet. "We all love each other…"

"You always think you're better than the rest of us…" Ichimatsu mumbled quietly, as he continued to pet Jyushimatsu's hair.

"I….I…." Choromatsu was at a lost for words. "I didn't want him to leave… I just didn't want him to say… to say the…."

"The _truth_?" Karamatsu spat looking at Choromatsu angrily. "You have to stop lying when it comes to things like this and instead of yelling hurtful things like that to Aniki, you could have waited until he cooled off."

Ever since Osomatsu's near suicide incident, Karamatsu had been even more protective than before. He was always ready to help Osomatsu when he needed it and always enjoyed spending time with the other eldest in the household.

"But…" Choromatsu began, before he was cut off by Todomatsu.

"What started the argument?" The youngest questioned glancing around for an answer. Jyushimatsu averted his eyes, making his own doe eyes narrow down on the fifth child. Jyushimatsu was always bad at keeping secrets anyway. "Jyushimatsu-niisan, do you have an answer?"

The yellow-Matsuno flinched back and he buried his face into Ichimatsu's shirt, the trashy feeling Matsuno rubbed his back comfortingly, while Choromatsu glared at Jyushimatsu's back and Karamatsu smacked the third eldest on the back of the head.

"Jyushimatsu-niisan, tell me what happened." Todomatsu said in a dead serious voice. "Or, I'll tell everyone what you said you felt about Ichimatsu-niisan last week."

Jyushimatsu's face turned red as he looked at Todomatsu, fear and anxiety clear on his face. He shook his head as confliction flashed across his face as he looked at his brother.

"T-The…" He began his usually steady voice shaking slightly. "The argument was about…"

Ichimatsu lightly rubbed the younger Matsuno's back in a comforting kind of way, making the other relax slightly.

"They were arguing about Osomatsu-niisan's position as older brother." He finished quietly, as he curled in on himself a bit. "Please don't be mad at me Choro-"

He was cut off as Choromatsu angrily slapped him across the face. The usually sunny smile was off his face and he was crying harshly, his left cheek had a red welt on it.

"You shouldn't have said anything Jyushimatsu!" Choromatsu said, face red with anger. Karamatsu quickly restrained the third eldest and dragged him after from the youngest trio.

"And you shouldn't have hit him!" Todomatsu shot back, as Ichimatsu growled low in his throat.

The door shut at his last remark.

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That night at dinner, Osomatsu never came home and nobody talked at the table either. But, they all had one thing running through their heads.

' _Where's Osomatsu-niisan?'_ Ichimatsu thought as he looked around the room.

 _ **(A/N: They're using that system they used in episode 23 where they were communicating without actually talking.)**_

' _I wish I knew…'_ Todomatsu thought cast Choromatsu a dark glance. The third brother looked at his beef stew which tasted bitter in his mouth.

' _I hope he's okay…'_ Choromatsu thought glancing around at all his brothers. ' _I didn't mean it… I was just so mad…'_

The room was chilly and silent as the brothers all stared at each other. Their mother walked into the room, hands placed onto her chest in a concerned fashion.

"Jyushimatsu." She spoke in a soft voice, as her fifth son looked up at her curiously. "Doctor Dekapan is on the phone for you."

The yellow parka wearing brother made his way to the living room, he saw the phone lying on it's side, waiting for him to pick it up.

"Hai, Jyushimatsu here." He said into the phone, his tone not as cheery as it usually was.

"Ah, Jyushimatsu, I have your brother Osomatsu-san here." The old doctors voice replied back to him. "I need you or some other brother to come and collect him."

The fifth child made a noise of affirmation before a question popped into his usually blank head. He made a confused noise as he waited for the doctor's own confirmation that he could ask the question.

"Why can't Osomatsu-niisan just come back by himself?" He found himself saying, without thinking of a better way of saying it.

"There was an… **accident** , with one of my latest experiments and your brother got caught in the middle of the cross fire, so he needs someone to come pick him up please, hoi." Jyushimatsu felt his heart freeze in his chest.

"I'll be there in a moment." He murmured hastily into the phone as he quickly hung up to go and get his slippers on.

Without even glancing at his brothers, he was out the door ignoring their confused voices calling out for him to stop and answer them.

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Arriving at Dekapan's Office in record time, he entered the establishment with a murmur of hurried greetings.

"Dekapan-hakase, where's my brother?" He asked, praying to every god, **even Kamimatsu** , that his dear eldest brother was okay.

"One second, hoi." The kind man said as he went to another part of the office. "Come on out, no one's going to hurt you… Jyushimatsu's here like I promised."

The meek shuffle of feet a few minutes later, showed the man had been successful in get his brother to come out.

Jyushimatsu felt his smile freeze on his face and then fall at what he saw in front of him. Standing there in a big red hoodie and a sleeve covered hand in Dekapan's was his older brother, who was now somewhere between four or five.

"What… happened to him?" Jyushimatsu questioned quietly, as he kneeled down in front of the smaller Matsuno careful not to startle him.

"Like I said, there was an accident, hoi." The old man replied. "He reverted to the state of a four or five year-old. He also doesn't seem to remember anything about his life, so I told him he had five older brothers. I told him you were going to come and get him, so he was expecting you."

"A-Are you m-my older brother?" The shy question hit the air and Jyushimatsu's head snapped back to the now child Osomatsu.

' _This must be before we were troublemakers of Akatsuka Town.'_ He thought remember when they all phased through the shy personality before finding their place in the town.

"Yeah, I'm Jyushimatsu." He answered picking the child up. Osomatsu laughed briefly as the physically older brother, spun him gently before placing him on his shoulders.

The five year-old _**(A/N: I've decided on five for his age.)**_ leaned against the slightly messy hair and began to fiddle with it, gently plucking the single cowlick on his head making him laugh.

"I'll be taking him home now." Jyushimatsu said as he looked at Dekapan nodded.

"I've already informed your mother, hoi."The older male said as he began to pack up his office. "She should have somethings read for your brother when you come home."

He nodded in understanding before standing, and bowing, making the child hold onto his head as to not tumble off.

"Thank you for holding him until now, and sorry for the trouble he probably caused before the accident." He apologized for his older brother like usual.

"Ah, he didn't cause trouble," The man corrected. "In fact **I** should be apologizing."

"What exactly happened anyway?" He asked standing upright and wincing when Osomatsu clutched his hair as he fell slightly backwards.

"Well, I was working on a age reducer pill, when Osomatsu-kun came in here looking upset." He began, as he arranged some medicine in a cabinet. "I offered him an ear and a cup of tea and he accepted."

Jyushimatsu swung the child back and forth on his oversized sleeves before throwing him up and catching him easily, making the adult turned child laugh childishly.

"I was going to test the pill on myself but, I ended up putting it into Osomatsu-kun's tea instead of my own… so this in reality is my fault…" The man ended with a quiet sight. "I'm really sorry Jyushimatsu-kun."

Jyushimatsu shook his head, as his hands continued to occupy the child relentlessly tickling him every now and then before moving out of grabby hands way, subconsciously as he looked at the doctor eyes serious and concerned.

"It's fine." He murmured. "Just please look for a way to reverse the effects."

The good doctor nodded, and Jyushimatsu bid him a goodnight before heading on his way.

' _How am I supposed to explain this to the others now?'_ He thought as the child in his arms looked at all the sights in front of him and around with wide sparkling eyes. ' _Man, things just went from bad to worst…'_

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 _Author's Note: Yay! Chapter 1 done! I'm going to continue this story along with my other stories. It's so much fun working with AU's for Osomatsu-san._

 _Don't forget to read and review. This story along with all of my other Osomatsu-san stories are on Archives of Our Own._

 _Bye-Bii!_


	2. Chapter 2

Chapter 2: Returning Home:

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 _Author's Note: Okay, so people are liking the story already I hope. It's really fun for me to write this cute side story away from all of my drama filled main stories, especially considering my genre I tend to stay in._

 **Chapter Summary: Jyushimatsu brings Osomatsu home. What do his brothers say? How do they react? Will everything be okay?**

 _Not too much to talk about today, so I'm gonna get along with it in the disclaimer now!_

 **Disclaimer: I don't own anything. All credit goes to original owners.**

 **Enjoy the Story!~ ?** **￢ﾝﾤ❤**

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Jyushimatsu opened the door as quietly as he could, considering how late it was. Looking down at the small child in his arms he giggled as he noticed Osomatsu was in and out of a sleepy state. Constantly rubbing his eye with a small fist and blinking to try and stay awake. He cradled the physically younger boy in his sleeve covered arms gently.

"Jyushimatsu?" He heard his mother's voice as she popped her head into the living room. 'Oh, good you're home. The others were getting restless."

He nodded silently. Then suddenly, as if remembering a sudden problem, he lifted the sleepy Osomatsu, dressed only in his, now, oversized red hoodie. His mother's gaze wandered over the smaller version of her eldest son with something of pity and longing before she gently took the child from the second youngests arms.

"You can't hold children under the armpits, Jyushi." She reprimanded him gently. "You have to make sure he's held comfortably."

She shifted the five year-old so he was positioned on her hip and placed his dozing head on her shoulder, as he lifted his thumb up and began to suck on it in.

Jyushimatsu watched his mom work with careful eyes, he didn't want to hurt the fragile version of his brother but, to be honest… he didn't know what he was doing.

Todomatsu was the only younger brother he got, and they were the same age, and Todomatsu was practically older than him! …. Maybe, their birth rights were switched and he was supposed to be youngest?

He shook his head. Thought's for another time. Right now, he needed to focus so he could learn how to better take care of his brother.

"Come on Jyushi, let's go give him a bath." She said. "Go in your room and open the closet, there should be some of your old clothes in there. Bring me the box."

He nodded his head obediently. He jogged quietly upstairs, and entered his room. The minute he did, he regretted it because, his other four brothers were soon looking at him confusion and worry clear on their faces. Karamatsu was the first to break the silence.

"Jyushimatsu, is Aniki okay?" The second eldest questioned as he looked at the fifth youngest. Jyushimatsu hesitated before nodding his head.

"He's fine…" He murmured. "Mom told you Dekapan-hakase called me because of Osomatsu-niisan, right?"

The others nodded, and he could tell that even with his words, they were worried. He bit his bottom lip in thought, as he weighed his options.

"...He was in a slight accident in Dekapan-hakase's office… The doctor has already told me that it was his fault and that he's deeply sorry. But, he's working on an antidote." He began his voice quiet but, loud in the quiet room.

"What's wrong with Aniki?" Karamatsu asked as he took a step forward, concern lacing his every step.

Jyushimatsu opened his mouth to respond when their mother entered the room with Osomatsu in a tanooki onesie and a red blanket in hand.

"I gave him a bath and something to eat," She spoke looking at Jyushimatsu only, seemingly not noticing her other four sons in the room. "He should sleep well tonight. He's not an infant so, unless nightmares strike him like they used to, you should be fine. If the others scare him too much than you should sleep next to him."

Jyushimatsu nodded his head as he looked at his mother, trying to process everything at once. He was going to be able to do this, he could take care of his older brothers baby form.

"M-Mom… is… is that…." Karamatsu seemed lost for words as he stared at the baby in shock.

"O-Osomatsu-niisan…?" Ichimatsu gaped as he looked at the baby that looked just like they all had at young ages.

The tired child looked over at the purple before walking over to him. His sleepy eyes, held confusion in them before they lit up with recognition.

"Are you Ichimatsu-niisan?" He asked in a soft tone. When the other Matsuno nodded, the young child lifted his arms, asking to be picked up.

Shakily he lifted the child, scared to drop him or scared that this wasn't a dream, no one was quite sure which. Soon, he was holding the child face to face, Osomatsu buried his face into the crook of the shocked males shoulder and neck and nuzzled it gently with a content noise.

"I told you Ichimatsu-niisan was really nice." Jyushimatsu's voice rang from the doorway. "He's really nice, just shy about his feelings."

The young child nodded once before, another content noise escape his mouth and soon the child was asleep in Ichimatsu's arms.

"The accident in Dekapan-hakase's office was when he accidentally switched their teas." Jyushimatsu explained after everyone had taken a seat.

Ichimatsu still had the child on his lap as he slept peacefully, he was holding him tightly as if he couldn't believe that it was really his older brother. Choromatsu seemed unable to tear his eyes away from the small toddler, while Todomatsu focused on his lap, quietly fiddling with his phone. And Karamatsu, the second eldest seemed to be taking this better than anyone. He was staring at Jyushimatsu with an intense look in his eyes as his brother proceeded to tell the tale of where the little Osomatsu came from.

"And, we just have to take care of him until Dekapan-hakase creates an antidote." He finished looking down at the toddler clinging onto Ichimatsu.

"...Okay, I guess we're going to have to be in charge for a little now, Choromatsu." Karamatsu said as he stood up and looked at their mother. "You don't mind helping us a bit do you mama?"

"Of course my little NEETs." Their mother smiled at them gently.

"Okay. Well, let's go to sleep for now." Karamatsu nodded once as he stood up and began to head to the futon.

The minute Ichimatsu stood up, Osomatsu squirmed in his grasp and began to make a low keening softly as he began to wake unpleasantly. Ichimatsu panicked, he had no idea what to do when a child began to cry.

Jyushimatsu took the child and rocked him, just how his mother taught him the minute he walked in.

Soon, Osomatsu fell back to sleep with a small smile on his face as he bunched up pieces of Jyushimatsu's shirt in his small fists.

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 _Author's Note: Chapter 2 is finally complete! I feel so proud, this is the third chapter this day that I'm posting and I feel very happy about this. Keep in tuned._

 _Just follow or subscribe to the story for instant updates on the stories._

 _Well, my adorable fans,_

 _Bye-Bii~~_


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